How hot will Earth be by 2100? The answer’s in the clouds

Understanding clouds is key to climate projections, but we don't understand …

Understanding the behavior of clouds is key to any accurate projection of future climate (as we’ve covered in the past) because of the complex, competing effects that clouds have on Earth’s energy budget.

Radiation from the sun in the form of visible light (which climate scientists refer to as "shortwave radiation") is reflected back into space by clouds. An increase in clouds (caused by increasing evaporation that comes with higher temperatures) will thus decrease temperatures by reducing incoming shortwave radiation. We call that a negative feedback—it acts to keep temperatures stable.

But water vapor is also a strong greenhouse gas. In fact, compared molecule for molecule, water vapor absorbs 100 times more outgoing infrared (or "longwave") radiation than does carbon dioxide. That means increasing evaporation will cause temperatures to increase even more. That’s a positive feedback—it acts to exacerbate rising temperatures.

(Despite its greenhouse potency, water vapor provides a feedback, not a forcing—it can’t initiate temperature increases because its presence in the atmosphere depends on temperature, unlike carbon dioxide or methane.)

If you want to project future climate change, you need a solid handle on how these competing feedbacks balance. What are the conditions where increasing evaporation will lead to more cloud formation? How much will cloud formation increase? Will it balance out the greenhouse effect of the water vapor?

These aren’t easy questions to answer, and this uncertainty accounts for much of the uncertainty seen in climate projections. Models that project a planet that is 1.7 to 4.4°C warmer by 2100 would give us a tighter range if we could model clouds more confidently.

Modeling problems

A recent paper in Geophysical Research Letters describes which facets of cloud behavior are giving the modelers problems; it does so by comparing the cloud feedbacks in twelve models cited in the most recent IPCC report. The researchers ran each model through to 2100 with the same emissions scenario and observed how the impact of clouds on Earth’s energy budget changed results by the end of the century.

First, the researchers considered the question of shortwave and longwave feedbacks. They found that the longwave feedback was pretty consistent across all models, and strongly positive (read: bad). The shortwave feedback was a different story—it ranged from a slightly negative to a strongly positive feedback and accounted for most of the difference among the models.

The authors also looked at the vertical distribution of clouds. Like the longwave feedback, high-level clouds showed a strongly positive feedback that was consistent across the models. Most of that positive feedback came from changes in the exact elevations of clouds, and not from changes in the amount or properties of those clouds. That's consistent with what’s known as the “Proportionately Higher Anvil Temperature”, or “PHAT,” hypothesis. (Don’t get that confused with the “Fixed Anvil Temperature”, or “FAT,” hypothesis…)

The differences among models concerned low-level clouds. The authors note that the biggest differences in low-level cloud behavior occurred in "regions associated with low-level subtropical marine cloud systems," which brings up an important point: there's a huge amount of geographical variation in these processes. The researchers aren't considering simplistic and abstract scenarios where all clouds will universally behave in a certain way; they're looking at very complex interactions of climatic factors that vary widely from place to place. Changes in cloud behavior over the equatorial Pacific look much different than those over central Europe. This is where global simulations shine—no scientist could possibly keep track of all the related interactions in their head.

So it looks like the biggest factor in climate model uncertainty is the shortwave feedback of low-level clouds. In fact, the authors get even more specific, pointing to "regions of subtropical subsidence and marine stratocumulus and trade cumulus clouds" as the biggest question mark.

Knowing what we don't know will help direct future research that aims to reduce uncertainty in climate projections. Considering that a recent study found that climate models may be underestimating the positive feedback from clouds and that temperatures could therefore end up above the projected range, it’s critical that we nail down the role clouds will play in climate change.

127 Reader Comments

I think the funniest aspect of climate change is that people are willing to trust scientists in every field of their lives, whether it be medicine, computer tech, radio tech, etc., but when it comes to something that requires a little sacrifice on their part for the sake of the planet and the world population the mindless greedy conservatives who've never contributed a thing to society go nuts.

On a more relevant note, Intel predicts having CPUs a million times faster than today by 2027 (and a 1000 times faster by 2018). I think that will allow for more complex simulations to determine even more accurate predictions of temperature increases.

"Despite its greenhouse potency, water vapor provides a feedback, not a forcing—it can’t initiate temperature increases because its presence in the atmosphere depends on temperature, unlike carbon dioxide or methane"

I am still trying to parse this - as far as I can tell it's utter nonsense as both CO2 and water vapor can be driven both by temperature and by other factors.

For example increased temperatures cause the oceans to outgas dissolved CO2 (see Vostok ice cores, where CO2 is shown to not initiate warming - the stanard interpretation of this data is that CO2 enhanced the warming - via guess what - feedback). On the flip side water vapor can be anthropogenic - via direct emission and land use changes.

Actually, if I read the article correctly, scientists currently have no clue the specific effect that clouds will have on the future climate. Maybe it'll be warmer, maybe cooler, maybe roughly unchanged. I'll say up front that *I* certainly have no clue about that either.

Yet the article seems to be written from a perspective predicated on an assumption of some apocalyptic change that needs to be anticipated and counteracted. In other words, "We know something bad is going to happen even though we don't know something bad is going to happen". As a person with an engineering degree, who has had a *lot* of science and math classes...that approach more closely represents a religious belief than a scientific method.

"Despite its greenhouse potency, water vapor provides a feedback, not a forcing—it can’t initiate temperature increases because its presence in the atmosphere depends on temperature, unlike carbon dioxide or methane"

I am still trying to parse this - as far as I can tell it's utter nonsense as both CO2 and water vapor can be driven both by temperature and by other factors.

For example increased temperatures cause the oceans to outgas dissolved CO2 (see Vostok ice cores, where CO2 is shown to not initiate warming - the stanard interpretation of this data is that CO2 enhanced the warming - via guess what - feedback). On the flip side water vapor can be anthropogenic - via direct emission and land use changes.

It is something of an odd statement; the article does a pretty good job of describing how complex the models are, and how much any given factor depends on numerous other factors, but then includes this somewhat blanket statement. I see the point, in that absent other factors, water vapor won't act as a forcing on its own due to its strong dependence on evaporation, while other greenhouse gases can potentially be released in ways that have nothing to do with temperature. It probably could have been worded better.

Actually, if I read the article correctly, scientists currently have no clue the specific effect that clouds will have on the future climate. Maybe it'll be warmer, maybe cooler, maybe roughly unchanged. I'll say up front that *I* certainly have no clue about that either.

Yet the article seems to be written from a perspective predicated on an assumption of some apocalyptic change that needs to be anticipated and counteracted. In other words, "We know something bad is going to happen even though we don't know something bad is going to happen". As a person with an engineering degree, who has had a *lot* of science and math classes...that approach more closely represents a religious belief than a scientific method.

Then you're not really reading the article correctly. We don't know exactly how much of a cooling effect clouds will have, because they're hard to model. We do know how much of a warming effect the increased water vapor will have:

Quote:

First, the researchers considered the question of shortwave and longwave feedbacks. They found that the longwave feedback was pretty consistent across all models, and strongly positive (read: bad). The shortwave feedback was a different story—it ranged from a slightly negative to a strongly positive feedback and accounted for most of the difference among the models.

Its more a matter of making the models more accurate, so that we know the correct amount of warming to plan for, rather than figuring out whether warming will occur at all.

Actually, if I read the article correctly, scientists currently have no clue the specific effect that clouds will have on the future climate. Maybe it'll be warmer, maybe cooler, maybe roughly unchanged.

Careful, you're setting up a false equivalence. Scientists *do* have an understanding. They also have error bars that show the limit of their understanding - as an engineer, I am sure you can appreciate that error bars do not mean clueless, but quite the opposite as they demonstrate in very specific terms the real limits of understanding.

Also, "maybe it'll be a little bit cooler, or maybe it will be a lotta bit warmer" would be a better way to phrase it. Your sentence almost alludes that there's a 1/3rd chance each that it could become warmer, cooler, or stay the same, and I'm pretty sure that's not the case.

Quote:

Yet the article seems to be written from a perspective predicated on an assumption of some apocalyptic change that needs to be anticipated and counteracted. In other words, "We know something bad is going to happen even though we don't know something bad is going to happen". As a person with an engineering degree, who has had a *lot* of science and math classes...that approach more closely represents a religious belief than a scientific method.

...I see no apocalyptic language in this post at all. I think you're letting your feelings regarding climate change color your reading experience. But if global temperature were to actually go up 4.4°C, I think that qualifies as apocalyptic.

We know something is happening. We know that it can become very, very bad. We don't see any evidence that it's becoming better. There are numerous studies with all kinds of numbers and qualified measures of uncertainty. I'm sorry, but I just don't see the religious aspect to this.

Its more a matter of making the models more accurate, so that we know the correct amount of warming to plan for, rather than figuring out whether warming will occur at all.

Whether there is warming once all effects are added up, does not stand to debate. One can roughly say that the increase CO2 would produce +1°C. This +1°C will cause higher atmosspheric H20 content, pushing temps additionally +1°C. These first +2°C do not stand to debate and were correctly estimated in the 70s.

All remaining effects (Land use, vegetation, methane emissions), besides of clouds, are of second order importance and contribute approx. +0.1-0.5°C.

Clouds have the largest uncertainty, ranging from -0.3°C to +2°C, with the sum of all scientific evidence pointing rather towards the +2°C than the negative value.

Let's not forget the real cause of all of this, the Sun. Get rid of the Sun and all the global warming problems will be solved.

Along with all other problems. Ever.

Ops31337 wrote:

I won't be here in 2100 as I'd be 137 so I don't care how hot it is.

Anyone who has kids (granted, not me yet) or cares about humanity in general (this does include me) likely finds your attitude to be quite irritating. And its not like temperatures are going to sit still until 2100, then jump up several degrees. The warming will be, and has been, gradual, so you'll see effects during your lifetime.

I have a great deal of trouble caring what the temperature will be in 2100 for two reasons: firstly because I won't be around and secondly because we can't predict what the weather will be like next week with any degree of certainty so claims of being able to predict the climate in 89 years time strikes me pure fantasy.

Interesting stuff, even if just a confirmation of what's already known.

Especially anyone who has spent any time in an area with high humidity.

Tho I wonder if understanding the science of prediciting the future of climate change is as important as the science behind controlling the effects in a manner that is both politically, economically and socially acceptable and responsible.

No point in coming up with a better model for prediction if nobody is going to do anything about it.

Politicians need to sell it, money needs to be made, and people need to buy into it. That, I think, is the most complex part of climate change.

Finally a reasonable article about AGW! Have the seas just parted? Just prior to James Hansen's 1988 congressional testimony, an article about cloud density appeared that claimed models did them all wrong indeed:

After that, all such suggestions were rapidly ignored, now going on 30+ years!

The LA Times featured cold fusion in '89 before its debunking. Greens were aghast!“It’s like giving a machine gun to an idiot child.” – Paul Ehrlich (mentor of John Cook of the SkepticalScience blog, author of "Climate Change Denial")“Clean-burning, non-polluting, hydrogen-using bulldozers still could knock down trees or build housing developments on farmland.” – Paul Ciotti (LA Times)“It gives some people the false hope that there are no limits to growth and no environmental price to be paid by having unlimited sources of energy.” – Jeremy Rifkin (NY Times)“Many people assume that cheaper, more abundant energy will mean that mankind is better off, but there is no evidence for that.” – Laura Nader (sister of Ralph)

CLIMATEGATE 101: "For your eyes only...Don't leave stuff lying around on ftp sites - you never know who is trawling them. The two MMs have been after the CRU station data for years. If they ever hear there is a Freedom of Information Act now in the UK, I think I'll delete the file rather than send to anyone....Tom Wigley has sent me a worried email when he heard about it - thought people could ask him for his model code. He has retired officially from UEA so he can hide behind that." - Phil "Hide The Decline" Jones to Michael "Hockey Stick" Mann

I have a great deal of trouble caring what the temperature will be in 2100 for two reasons: firstly because I won't be around and secondly because we can't predict what the weather will be like next week with any degree of certainty so claims of being able to predict the climate in 89 years time strikes me pure fantasy.

Seems to me that this is more of a comprehensional error, than a factual one. Just to get you started..... if all we were talking about is weather.... we probably wouldn't bother using the term "climate" so much.

I think the funniest aspect of climate change is that people are willing to trust scientists in every field of their lives, whether it be medicine, computer tech, radio tech, etc., but when it comes to something that requires a little sacrifice on their part for the sake of the planet and the world population the mindless greedy conservatives who've never contributed a thing to society go nuts.

That's because you don't understand science. Science is about coming up with thesis and then *proving* them through experiment. Repeatedly.

Climatology is more akin to alchemy -it certainly doesn't qualify as science- because they are incapable of proving any of their ideas. Don't believe me? There have been major shifts in global temperature even in the last 3-4000 years: North Africa was once lush and fertile and you used to be able to skate and hold markets on a frozen river Thames. These changes in climate are well within the change that 'climatologists' claim is happening now but suddenly it is -without any experimental proof- all man made rather than it being a natural cycle.

I think the funniest aspect of climate change is that people are willing to trust scientists in every field of their lives, whether it be medicine, computer tech, radio tech, etc., but when it comes to something that requires a little sacrifice on their part for the sake of the planet and the world population the mindless greedy conservatives who've never contributed a thing to society go nuts.

And yet according to the doomsayers and man-made global warming freaks in the 1970s, we should all be dead by now.

I think the funniest aspect of climate change is that people are willing to trust scientists in every field of their lives, whether it be medicine, computer tech, radio tech, etc., but when it comes to something that requires a little sacrifice on their part for the sake of the planet and the world population the mindless greedy conservatives who've never contributed a thing to society go nuts.

That's because you don't understand science. Science is about coming up with thesis and then *proving* them through experiment. Repeatedly.

Climatology is more akin to alchemy -it certainly doesn't qualify as science- because they are incapable of proving any of their ideas. Don't believe me? There have been major shifts in global temperature even in the last 3-4000 years: North Africa was once lush and fertile and you used to be able to skate and hold markets on a frozen river Thames. These changes in climate are well within the change that 'climatologists' claim is happening now but suddenly it is -without any experimental proof- all man made rather than it being a natural cycle.

Climatology isn't science it's more akin to astrology.

I am not going to bother taking the time to dismantel and address all your claims. However, I would like for you to prove this:

Quote:

"There have been major shifts in global temperature even in the last 3-4000 years"

Your example would be local, and not global.

Quote:

North Africa was once lush and fertile and you used to be able to skate and hold markets on a frozen river Thames.

I think the funniest aspect of climate change is that people are willing to trust scientists in every field of their lives, whether it be medicine, computer tech, radio tech, etc., but when it comes to something that requires a little sacrifice on their part for the sake of the planet and the world population the mindless greedy conservatives who've never contributed a thing to society go nuts.

That's because you don't understand science. Science is about coming up with thesis and then *proving* them through experiment. Repeatedly.

Climatology is more akin to alchemy -it certainly doesn't qualify as science- because they are incapable of proving any of their ideas. Don't believe me? There have been major shifts in global temperature even in the last 3-4000 years: North Africa was once lush and fertile and you used to be able to skate and hold markets on a frozen river Thames. These changes in climate are well within the change that 'climatologists' claim is happening now but suddenly it is -without any experimental proof- all man made rather than it being a natural cycle.

Climatology isn't science it's more akin to astrology.

wow, really, is that the reason we are utterly incapable to find the oil/coal/gas reserves?

Unfortunately, the disbelievers do have a point in the attempted power grab by certain segments of the lefties, using AGW as the basis to push the ways they believe humans should live. There are just as many crazies opposing them, who would make everyone else's life a horror rather than give up a single privilege. I still haven't seen any reliable proof that AGW isn't real, and with population growth, only likely to get worse, but I do share some concern for the political responses.

Though you'd think people complaining about predicting weather would have noticed how it gets warm around the same time every year, and cold at certain times, and been able to draw some conclusions.

I think the funniest aspect of climate change is that people are willing to trust scientists in every field of their lives, whether it be medicine, computer tech, radio tech, etc., but when it comes to something that requires a little sacrifice on their part for the sake of the planet and the world population the mindless greedy conservatives who've never contributed a thing to society go nuts.

That's because you don't understand science. Science is about coming up with thesis and then *proving* them through experiment. Repeatedly.

Did you happen to have a few extra copies of the planet sitting around that we can run experiments on? No? Didn't think so. Climatology is a statistical science, yes, so we can't put absolute certainty on anything. However, if someone tells you the odds of something happening are over 90%, and there's over 95% certainty regarding the cause, you place your bets on the most likely scenario.

Theodophilus P. Wildebeeste wrote:

Climatology is more akin to alchemy -it certainly doesn't qualify as science- because they are incapable of proving any of their ideas. Don't believe me? There have been major shifts in global temperature even in the last 3-4000 years: North Africa was once lush and fertile and you used to be able to skate and hold markets on a frozen river Thames. These changes in climate are well within the change that 'climatologists' claim is happening now but suddenly it is -without any experimental proof- all man made rather than it being a natural cycle.

So lets see, North Africa used to be fertile farmland, and it is now a desert. The Thames used to freeze over, and now it doesn't. Despite your use of very local examples, you're still giving evidence that it is now warmer than it used to be. Climatology helps us figure out why that is. Known natural forcings wouldn't cause the warming we've seen, while human activity does:

"Despite its greenhouse potency, water vapor provides a feedback, not a forcing—it can’t initiate temperature increases because its presence in the atmosphere depends on temperature, unlike carbon dioxide or methane"

I am still trying to parse this - as far as I can tell it's utter nonsense as both CO2 and water vapor can be driven both by temperature and by other factors.

For example increased temperatures cause the oceans to outgas dissolved CO2 (see Vostok ice cores, where CO2 is shown to not initiate warming - the stanard interpretation of this data is that CO2 enhanced the warming - via guess what - feedback). On the flip side water vapor can be anthropogenic - via direct emission and land use changes.

It is something of an odd statement; the article does a pretty good job of describing how complex the models are, and how much any given factor depends on numerous other factors, but then includes this somewhat blanket statement. I see the point, in that absent other factors, water vapor won't act as a forcing on its own due to its strong dependence on evaporation, while other greenhouse gases can potentially be released in ways that have nothing to do with temperature. It probably could have been worded better.

I apologize for any confusion. You are correct to say that carbon dioxide is also affected by feedbacks. I didn't mean to get semantic. However, greenhouse gases are typically described as forcings, and water vapor is considered a feedback. The equilibrium of the hydrologic cycle stays pretty close to temperature. It's difficult to perturb it for very long. The carbon cycle, though, balances on a massively longer time scale, so the equilibrium concept doesn't really apply. Greenhouse gases have plenty of time to change temperatures before feedbacks bring them back to the "original" conditions.When climate scientists look for causes of climate change in the past, they look at solar radiation and greenhouse gases (for example), not water vapor.

NikFromNYC wrote:

Just prior to James Hansen's 1988 congressional testimony, an article about cloud density appeared that claimed models did them all wrong indeed:

After that, all such suggestions were rapidly ignored, now going on 30+ years!

Perhaps you should take a look through the references for the paper this article is about and see how many studies on cloud feedback have been performed since 1987 (almost 30 cited in this paper alone). You should also look at the paper I linked in the last paragraph. It's a bit more sophisticated (technically) than the 1987 paper you cited, and comes to precisely the opposite conclusion.

Thanks for the clarification, Scott. The subject is not an easy one, and its hard to get the information across without getting too verbose or referencing naming conventions among scientists. Considering the number of words I've typed trying to educate people in the comments, and the amount of time I've spent correcting people on the use of such basic words as "theory", I can sympathize.

Doesn't surprise me, actually. The timeline fits with the growth of the Chinese and Indian economies, and as the linked article notes, it seems like pretty much the same thing that happened between the 1940s and 1970s. Of course, sulfate aerosols aren't a good thing to put in the air, which is why we cut back on that.

I have a great deal of trouble caring what the temperature will be in 2100 for two reasons: firstly because I won't be around and secondly because we can't predict what the weather will be like next week with any degree of certainty so claims of being able to predict the climate in 89 years time strikes me pure fantasy.

It is vastly easier to predict the average global temperature in 2100 than it is to predict the temperature at my residence tomorrow afternoon at this exact same time. We have more certainty about the number of baskets LeBron James will make all season three years from now than we do about whether he will make any *single* given shot that he takes in any individual game that takes place in the next few months.

I have a great deal of trouble caring what the temperature will be in 2100 for two reasons: firstly because I won't be around and secondly because we can't predict what the weather will be like next week with any degree of certainty so claims of being able to predict the climate in 89 years time strikes me pure fantasy.

It is vastly easier to predict the average global temperature in 2100 than it is to predict the temperature at my residence tomorrow afternoon at this exact same time. We have more certainty about the number of baskets LeBron James will make all season three years from now than we do about whether he will make any *single* given shot that he takes in any individual game that takes place in the next few months.

Well put. Statistical sciences require one to understand the basic concepts of statistics. I can't predict the result of a given coin toss, but over a long series of coin tosses, I can predict with reasonable accuracy that approximately 50% will be heads. There's a chance I'm wrong, but the more data I collect (i.e. the longer the series of coin tosses), the more likely it is that I'll be right.

Weather also requires some understanding of chaos theory, but since the best real-world example of a chaotic system is weather, its harder to make a comparison. Referencing the gravitational N-body problem isn't usually helpful.

How did you get a PhD from an Ivy League school? You apparently think that "A bunch of misleading out-of-context quotes by a bunch of random non-scientists commenting about something almost completely unrelated to the issue at hand" is a sound argument. I'm not a PhD and didn't go to an Ivy League school and I'm nowhere near that stupid, so how are you managing it?

I have a great deal of trouble caring what the temperature will be in 2100 for two reasons: firstly because I won't be around and secondly because we can't predict what the weather will be like next week with any degree of certainty so claims of being able to predict the climate in 89 years time strikes me pure fantasy.

This has to be about the single dumbest argument advanced by global warming "skeptics" (who often seem to be remarkably credulous when it comes to foolish arguments that support their bias). After all, if lack of short term predictability implied lack of long term predictability, then casinos would constantly be going broke,